[PATCH 2/2] ARM: rust: Enable Rust support for ARMv5TE

[Karl Mehltretter <kmehltretter@gmail.com>]

HAVE_RUST is currently selected for CPU_32v7 only. The
arm-unknown-linux-gnueabi target used for ARMv7 emits ARMv6
instructions (+v6), so it cannot be used for ARMv5. Use rustc's
built-in armv5te-unknown-linux-gnueabi target instead. It keeps the
Linux EABI (no short enums, unlike the -none-eabi targets) and is
soft-float and strict-align.

Kernels that also contain ARMv4 or ARMv4T CPUs are built for the
lowest architecture and stay excluded. CPU_32v5 also covers the ARMv5T
ARM1020. C code is already built with -march=armv5te there, so Rust
matches.

The kernel crate implements atomics through the C helpers rather than
core::sync::atomic, so the missing atomic instructions on ARMv5 do not
matter. The only gap was the 2-byte xchg() added by "ARM: cmpxchg:
support 2-byte xchg() on pre-ARMv6 CPUs".

core::sync::atomic types up to 32 bits still compile on this target,
but LLVM lowers them to __sync_* libcalls which the kernel does not
provide. Nothing in the tree uses them. Such code would fail to link
on ARMv5 while it links on ARMv7.

Tested on v7.3-rc1 with clang 22.1.8, rustc 1.99.0 and bindgen 0.73.2
in QEMU versatilepb and on a Microchip SAM9X75 Curiosity board (both
ARM926EJ-S). The Rust samples, all Rust KUnit suites and the doctests
pass.

Link: https://lore.kernel.org/rust-for-linux/CACRpkdYF0sVB2-qgy=GzETSR3+2sagVQPGdunDQDJrn8KqJorA@mail.gmail.com/
Link: https://lore.kernel.org/rust-for-linux/b13d37bd-ec68-4713-94e5-e9ed4d6a6354@gmail.com/
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 Documentation/rust/arch-support.rst | 2 +-
 arch/arm/Kconfig                    | 2 +-
 arch/arm/Makefile                   | 4 ++++
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index 4f980815e92a..2193d8e1e156 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -15,7 +15,7 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 =============  ================  ==============================================
 Architecture   Level of support  Constraints
 =============  ================  ==============================================
-``arm``        Maintained        ARMv7 Little Endian only.
+``arm``        Maintained        ARMv5TE and ARMv7, Little Endian only.
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        \-
 ``riscv``      Maintained        ``riscv64`` and LLVM/Clang only.
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ffbc7f386131..f6b14d9f0e1f 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -138,7 +138,7 @@ config ARM
 	select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RSEQ
-	select HAVE_RUST if CPU_LITTLE_ENDIAN && CPU_32v7 && !KASAN
+	select HAVE_RUST if CPU_LITTLE_ENDIAN && (CPU_32v7 || (CPU_32v5 && !CPU_32v4T && !CPU_32v4)) && !KASAN
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UID16
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 573813ef5e77..ef94eae1f047 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -150,7 +150,11 @@ endif
 KBUILD_CPPFLAGS	+=$(cpp-y)
 KBUILD_CFLAGS	+=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
 KBUILD_AFLAGS	+=$(CFLAGS_ABI) $(AFLAGS_ISA) -Wa,$(arch-y) $(tune-y) -include $(srctree)/arch/arm/include/asm/unified.h -msoft-float
+ifdef CONFIG_CPU_32v5
+KBUILD_RUSTFLAGS += --target=armv5te-unknown-linux-gnueabi
+else
 KBUILD_RUSTFLAGS += --target=arm-unknown-linux-gnueabi
+endif
 
 CHECKFLAGS	+= -D__arm__
 
-- 
2.39.5 (Apple Git-154)